Charged particle carrier transport in a conventional solid state transistor is dominated by a drift-diffusion mechanism, with an associated transport velocity limit of about 5×107 cm/sec. A vacuum channel, when constructed, will rely upon thermionic emission and quantum tunneling, with a theoretical ballistic transport velocity of about 3×1010 cm/sec. In conventional semiconductor action, electrons are scattered, and high temperature operation often results in reduction of drive current. Electrons in vacuum can move with few or no collisions, thereby increasing average electron velocity.
A conventional solid state semiconductor transistor does not perform well in extreme environments, such as very high temperature and/or where radiation is present. Vacuum devices offer immunity to radiation, increased robustness and relatively high frequency. A vacuum channel transistor might operate ballistically for carrier transport, if some operating conditions can be modified. However, this would require use of smaller transport distances, among other characteristics.
Early electronics centered use of a vacuum tube to amplify, switch, or modulate electrical signals. Many decades ago, vacuum tubes were replaced by solid-state devices such as the metal-oxide-semiconductor field-effect transistor (MOSFET) and diode. Nevertheless, vacuum tubes are still used in niche applications such as premier sound systems and high-power radio base stations.
The transition from the vacuum tube to the solid-state device was not driven by the superiority of the semiconductor as a carrier transport medium, but by the ease of fabrication, low cost, low-power consumption, lightness, long lifetime, and ideal form factor for integrated circuits (ICs). The vacuum tubes were fabricated by mechanical machining and used as discrete components, whereas modern solid-state devices are batch processed in assembling the integrated circuits. The vacuum device is more robust than solid-state devices in extreme environments involving high temperature and exposure to radiation. The critical tradeoff is that vacuum tubes yield higher frequency/power output but consume more energy than MOSFETs. Transport in a vacuum is intrinsically superior to transport in a solid medium, because vacuum transport allows ballistic transport while solid state carriers suffer from optical and acoustic phonon scattering in semiconductors.
Charged particle carrier transport in a conventional solid state transistor is dominated by a drift-diffusion mechanism, with an associated transport velocity limit of about 5×107 cm/sec. A vacuum channel, when constructed, will rely upon thermionic emission and quantum tunneling, with a theoretical ballistic transport velocity of about 3×1010 cm/sec.
Cathodes of conventional vacuum tubes need to be heated for thermionic emission of electrons, and the energy for heating adversely overwhelms the energy required for field emission. A conventional vacuum device is, therefore, not suitable for low power devices. For high power amplification (e.g., >50 W), however, the solid state device needs a complex circuit architecture including many transistors, microstrips, and thermal management systems.
Advantages of both devices can be achieved together if the macroscale vacuum tube is miniaturized to a nanometer scale. A nano-vacuum tube can provide high frequency/power output while satisfying the metrics of low mass, small size, modest cost, lifetime, and stability in harsh conditions.
More importantly, further downscaling can allow use of a cold cathode, because the electric field itself is strong enough to emit electrons. Also, an ultimate downscaling combined with low work function materials may decrease the turn-on gate voltage and drain voltage to less than 1 Volt, thus enabling these devices to be competitive with modern semiconductor technology. These benefits can be attained by use of matured IC technology to fabricate nano scale vacuum tubes and to facilitate circuit integration.
The most common design of vacuum microelectronics is a vertical field emitter consisting of the emitter, gate, and collector. The emitter is a sharp conical tip, the gate is a circular aperture, and the collector is flapped at the top. The movement of electrons between the emitter (cathode) and the collector (anode) is controlled by the gate.
An array of vertical field emitters forms a large-area flat electron source. Unfortunately, the vertical structure may be undesirable for circuit implementation due to the difficulties in achieving geometrical dimensions with identical gap spacing over all devices on the substrate. In contrast, the geometry of a planar structure is defined by photolithography enabling practical integration.
However, as the distance between the emitter and the gate shrinks, processing becomes difficult. In addition, a fraction of the emitted electrons can be easily swept into the gate, and electrons at the gate can be emitted to the collector, both of which are detrimental in circuit design.